Biomedical Engineering Reference
In-Depth Information
One common means of characterizing the transverse relaxivity of SPIO is to
report
R
1
and
R
2
. For these particles, the addition of a complementary target DNA
sequence led to a change in
R
2
, from 75 s
− 1
m
M
− 1
to 128 s
− 1
m
M
− 1
, but no change
in the
R
1
of 27 s
− 1
m M
− 1
. A linear dependence of measured T
2
values on the amount
of nucleic acid analyte was reported (Figure 1.8), which was consistent with the
above-discussed observations of particle size and relaxivity [20, 61]. Josephson
et al.
were able to detect tens of femtomoles of DNA in 1 ml, and suggested that
amounts as low even as attomoles might be detected. This sensitivity was realized
three years later when by tens of attomoles of DNA oligonucleotides were detected
in a volume of 50
l (0.2 p
M
concentration) after a incubation time of 40-60 min
and a change in T
2
of 30 ms [66] . Grimm
et al.
compared the sensitivity and dose
response of their MRSws to a standard telomeric repeat hybridization assay, which
was a PCR-dependent, ELISA-based photometric assay. For their 54-mer telomeric
repeat test sequence, these authors showed a very tight correlation (
r
2
= 0.99) and
equivalent sensitivity between the two methods. Grimm
et al.
concluded that the
performance of MRSws matched that of PCR-independent assays, and was within
the upper range of PCR-based assays. Additionally, MRSws have advantages over
other DNA assays in that they are inherently quantitative, quick and simple to run,
have no requirement for a solid phase, and inherently lack the PCR-related arti-
facts [66] .
μ
Figure 1.8
First demonstration of a magnetic
relaxation switch biosensor. On the addition
of femtomoles of single-stranded
oligonucleotide the T
2
value was decreased by
20 ms due to nanoparticle clustering (
). No
change in T2 was observed in the absence of
complementary oligonucleotide (
). T
2
values
responded linearly with increasing amounts of
analyte (inset). Original fi gure provided by Dr
Lee Josephson, Center for Molecule Imaging
Research, Massachusetts General Hospital,
Boston, MA. Reproduced with permission
from Ref. [1]; © 2001, Wiley-VCH Verlag
GmbH & Co. KGaA.
